1. Field of the Invention
The present invention relates to a technique for controlling the driving force of a vehicle with consideration for vehicle slip.
2. Description of the Related Art
Hybrid vehicles have been developed in which internal combustion engines and motors are provided as prime movers. A technique for controlling the slip of a hybrid vehicle is disclosed, for example, in JPA 10-304514. According to this technique, drive wheels become more likely to slip, prompting a reduction in motor torque, when the rate of change of the rotational angular speed (also referred to as xe2x80x9cangular accelerationxe2x80x9d) of a drive shaft exceeds a threshold value. It is thus possible to prevent slip from occurring during an increase in the driving force of a motor.
The above-described slip control technique entails reducing the motor torque once the drive wheels have slipped, but the subsequent reduction in rotational angular speed allows the motor torque to be increased, occasionally leading to another slippage event. Slippage recurs in such cases. In particular, hybrid vehicles tend to be susceptible to this phenomenon because of the fast motor response to a varying torque. This shortcoming is not limited to hybrid vehicles and extends to other types of vehicles.
Accordingly, an object of the present invention is to reduce the likelihood that the torque control of a drive shaft will result in repeated slippage.
In order to attain at least part of the above and related objects of the present invention, there is provided a slip control device for use in a vehicle, which includes at least one prime mover for driving a drive shaft of the vehicle. The drive shaft transmits a driving force of the prime mover to the wheels of the vehicle. The slip control device comprises an angular acceleration meter configured to measure an angular acceleration of the drive shaft, and a torque controller. The torque controller enhances torque restrictions of the drive shaft when the angular acceleration exceeds a specific first threshold value, and relaxes the enhanced torque restrictions once the angular acceleration falls below the first threshold value and further satisfies a specific restriction relaxation condition
According to the above structure, the likelihood of repeated slippage is reduced because the torque restrictions are relaxed only when the specific restriction relaxation condition is satisfied, but not merely when the angular acceleration of the drive shaft falls below the first threshold value.
In a preferred embodiment, the torque restrictions are expressed as a graph representing a relation between the angular acceleration and an upper torque limit of the drive shaft in a two dimensional space defined by an angular acceleration axis and a torque axis where the upper torque limit of the drive shaft decreases with increased angular acceleration. In this case, the torque restrictions are enhanced by moving a position of the torque axis relative to the graph along the angular acceleration axis while preserving a shape of the graph.
In another embodiment, the torque restrictions are enhanced and relaxed by varying a minimum value available for the angular acceleration in the graph. In still another embodiment, the torque restrictions are enhanced and relaxed by varying a maximum value available for the upper torque limit in the graph.
The present invention can be embodied in a variety of forms. Examples include vehicle slip control devices and methods, vehicles having such control devices, computer programs for executing the functions of such control devices and methods, recording media for storing such computer programs, and data signals embodied on a carrier wave including such computer programs.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.